TOI544b: a new small planet inside the radius valley.

TOI544b is a newly-characterised small planet (mass < 4 Earth masses) recently detected with TESS and followed-up by the KESPRINT consortium. High-precision RV observations have allowed the mass of the planet to be constrained within 15% uncertainty and detect a previously-unknown longer, 50 day, period companion with a mass precision of 10%, meaning these are some of the most precisely known exoplanet masses found using the RV method. The relatively low density of the inner planet puts it within a small subset of exoplanets which could be composed of mainly rocky silicates or water ices with or without layers of atmospheric hydrogen. However, the short orbital period means the effective temperature of this planet is above what would be expected for a rocky core to be able to sustain a hydrogen layer. We discuss the possibility of this planet joining the small group of ‘ocean worlds’ containing a significant fraction of water. As well as this, the planet sits firmly within the radius valley, a region where very few exoplanets are expected to be found. This, coupled with the high Transmission Spectroscopy Metric (TSM) and Emission Spectroscopy Metric (ESM) values, make TOI-544b an excellent target for future atmospheric characterisation with the recently-launched James Webb Space Telescope and future Ariel mission. Finally, the importance of the TOI-544 system in relation to the wider population of small exoplanets and our understanding of planetary compositions will be presented.